Processes for the synthesis of a hydrocarbyl bis(dihydrocarbyl phosphate) by the reaction of a hydrocarbyl diol with a dihydrocarbyl halophosphate in the presence of a Lewis acid catalyst are known to the prior art. For example, U.S. Pat. No. 3,254,973 to J. J. Giammaria et al. illustrates such a general reaction and employs nitrogen gas to sparge the reaction medium and remove hydrogen chloride as by-product. U.S. Pat. No. 4,133,846 to J. A. Albright illustrates an analogous reaction but does not show the nitrogen sparge aspect. More recent U.S. Pat. No. 4,343,732 to T. Zama et al. shows the reaction of certain diols, phosphorus oxychloride, and certain alcohols and phenols, in the presence of aluminum chloride as a catalyst, indicates (at Col. 4, lines 21-22) that inert solvents, such as toluene and xylene, may be used, but demonstrates no advantage for the use of a solvent.
When the diol used in the aforementioned reaction is alkylene in character, such as neopentylglycol, the reaction mixture comprising the desired diphosphate compound will generally contain undesired cyclic by-product(s), such as of the formula ##STR1## along with triphenyl phosphate,which lower the thermal stability of the diphosphate-containing compositions. If the diol which is used contains an alkylene group in association with phenyl moieties, such as in bisphenol A, the impurity will, for example, be isopropyl phenyl diphenyl phosphate. The unstable nature of such diols, in regard to the aforementioned formation of by-product(s), is due to either intramolecular reaction and rearrangement, such as in the case of the branched neopentyl glycol reagent and/or actual splitting apart of the diol molecule as in the case of bisphenol A which comprises two phenyl groups linked together with an alkylene moiety, namely --C(CH.sub.3).sub.2 --. The terminology "unstable hydrocarbyl-containing" will be used to characterize either type of such diols.